Pathogenic bacteria require iron for their survival and ability to cause infection. Heme comprises 90% of the iron available within the host. Therefore, understanding the mechanism of heme acquisition and iron release will provide the knowledge required for the development of new therapeutic targets. Both gram-negative and gram-positive pathogenic bacteria have evolved receptor mediated heme uptake systems by which they acquire iron. A key step in the process is the release of iron from the heme macrocycle by the action of heme oxygenase (HO). The specific aims of the proposal are to a) structurally characterize the soluble bacterial HO enzymes from C. diphtheriae (cd-HO), N. meningitides (nm-HO) and P. aeruginosa (pa-HO). The cd-HO is structurally homologous to the mammalian HO proteins and will serve as a model system for the larger membrane bound proteins. The nm-HO and pa-HO represent a new unique class of HO enzymes that in the case of pa-HO show an altered regioselectivity. Using both X-ray crystallographic and NMR methodologies we will be able to obtain significant insight into the role of protein conformation and dynamics in heme reactivity in this unique family of enzymes; b) elucidate the mechanism of heme hydroxylation and regioselectivity by a combination of site-directed mutagenesis and spectroscopic studies designed to identify key structural and electronic factors in determining both regioselectivity and the formation of a key intermediate in the HO reaction; c) To elucidate the mechanistic formation and biophysical properties of verdoheme and biliverdin. The elucidation of the mechanism of action of the bacterial HO proteins will be crucial in understanding the role of heme utilization in pathogenesis, as well as in future development of inhibitors as potential therapeutic agents.